Nowadays, many electronic devices incorporate functionality that operates at radio frequencies, such as mobile communication devices. The implementation of such functionality in a cost-effective manner is far from trivial. It is well-known that bipolar transistors are particularly suitable for handling signals in the radio frequency (RF) domain.
However, the manufacture of integrated circuits (ICs) based on silicon bipolar transistor technology is more costly than for instance complementary metal oxide semiconductor (CMOS) ICs, and the downscaling of the device feature size is more easily achieved in CMOS technology. The cost-effective nature of CMOS technology has led to the acceptance of CMOS technology as the mainstream technology of choice for the manufacture of a wide variety of semiconductor components including ICs.
Efforts have been made to produce bipolar transistors within a CMOS process flow, thereby providing mixed technology ICs in which bipolar transistors can be used for handling RF signals. Such process flows are sometimes referred to as BiCMOS technology. An example of a method of manufacturing a heterojunction bipolar transistor (HBT) in a BiCMOS manufacturing process is provided in EP 2 466 628 A1, in which the heterojunction bipolar transistor has a SiGe base (SiGe HBT).
Transistors suffer from the well-known Johnson limit. For a bipolar transistor, this limit equates to the product of peak current gain cut-off frequency fT, which is the high-frequency figure of merit of the transistor, and the collector-emitter breakdown voltage BVCEO. In other words, it is far from trivial to provide a bipolar transistor that can handle both high operating frequencies as well as high voltages. These characteristics are typically obtained by controlling the amount of doping in the collector. A high collector doping level on the one hand increases fT because it postpones the Kirk effect but on the other hand decreases BVCEO because it increases the local electric field, and thus the avalanche multiplication.
The high-frequency order of merit figure fT, the collector-emitter breakdown voltage BVCEO and the collector-base junction breakdown voltage BVCBO of a bipolar transistor all depend of the collector doping Nc. The maximum of the fT curve is determined by the start of the Kirk effect, which occurs at a collector current density Jk. This is shown in FIG. 1, which demonstrates that the higher the doping concentration Nc, the higher Jk and the peak value of fT become.
BVCEO is the voltage at which the hole-current generated by avalanche is sufficiently large to keep the emitter-base junction forward biased, such that a transistor current remains in the absence of an external base current. BVCEO therefore increases when the avalanche decreases. The avalanche effect is due to the acceleration of the electrons caused by a high electric field in the depletion region.
It is well-known that the small region surrounding the point of maximum electric field provides the largest contribution to the avalanche current. The depth of the depletion region over which the electric charges can be distributed is inversely related to the doping level Nc in the collector. As shown in FIG. 2, a bipolar transistor having relatively low levels of doping in a collector 11 buried collector 20 causes the formation of a relatively large depletion region 15 at the interface between the collector 11 and a base 30, whereas a bipolar transistor having a relatively higher level of doping in a collector 11′ causes the formation of a relatively confined depletion region 15′ at the interface between the collector 11′ and a base 30. Hence, at high doping levels, the depletion region 15′ is condensed, which increases the maximum electric field and, consequentially, the avalanche current, thus lowering BVCEO. Similarly, BVCBO scales with 1/Nc and is therefore higher at low collector doping.
One way to increase the product of fT*BVCEO, thereby overcoming the Johnson limit, is to apply a field plate along the collector to decrease the electric field and therefore postpone the onset of the avalanche phenomenon to a higher voltage. This is known as the reduced surface field (RESURF) effect, and is schematically depicted in FIG. 3. The RESURF effect extends the depletion region 15 at the interface between the base 30 and the collector 11 towards the highly-doped collector 20 by applying a suitable potential to the field plate or gate 50.
Raymond J. E. Hueting et al. in “A New Trench Bipolar Transistor for RF Applications” in IEEE Transactions on Electron Devices, Vol. 51(7), 2004, pages 1108-1113 disclose a theoretical model of a vertical trench SiGe HBT having a trench field plate connected to the emitter and a linearly graded doping profile in the collector drift region that demonstrates improved avalanche characteristics at higher voltages. The field plate is electrically isolated from a SiGe base and the collector by a 135 nm thick oxide layer. This article further shows in FIG. 6 that electrical connection of a field plate to the emitter instead of to the base gives a further improvement in the peak cut-off frequency fT of a HBT.
It is however far from trivial to manufacture such a device in a cost-effective manner, especially in a BiCMOS process.